Table 1.
Existing studies addressing seasonal variation in glycemic control.
| Design | Number | Women (%) | Location | Winter A1C (%) | Summer A1C (%) | Winter-summer decline in A1C (%) | |
| LETTER | |||||||
| Carney et al, 2000 [12] | Retrospective cohort study using clinic records | 2,080 | Not reported | Northumberland (United Kingdom) | 8.4 | 7.7 | 0.6 to 0.7 in women only |
| Asplund, 1997 [11] | Retrospective cohort study using clinic records | 800 | Not reported | Sweden | 7.61 | 7.23 | 0.38 |
| Ishii et al, 2001 [8] | Prospective cohort study with monthly A1C | 39 | 69 | Fukushima province, Japan | 6.96 ± 0.9 | 6.42 ± 0.65 | 0.54 |
| Sohimiya et al [9] | Prospective cohort study with monthly A1C | 11 | 0 | Japan | 6.65 ± 0.12 | 6.25 ± 0.21 | 0.40 |
| ARTICLE | |||||||
| Chen et al, 2004* [10] | Prospective cohort study | 110 | 25 | Taiwan | 7.53 ± 1.37 | 7.31 ± 1.29 | 0.21 ± 0.94% |
| Tseng et al, 2005 [13] Temp (°C): | Retrospective cohort study using database | 285,705 veterans. Tests: | 2.2 | United States, various areas | 8 | 7.8 | |
| >10 | 117,665 | 0.07 | |||||
| 4.4 to 10 | 204,001 | 0.08 | |||||
| 0 to 4.4 | 159,485 | 0.24 | |||||
| -6.7 to 0 | 229,239 | 0.16 | |||||
| -15 to -6.7 | 77,885 | 0.13 | |||||
*In contrast to other studies which assessed a winter-summer difference, this study was concerned with a late winter (February) vs. pre-winter (November) difference.